Numerous surfactant applications depend on the attainment of low surface tensions. Whereas conventional hydrocarbon surfactants can attain surfact tensions of as low as 23 dynes/cm, fluorinated surfactants are unique in that they can attain surface tensions of 15-20 dynes/cm, and at best of 14.5 dynes/cm. Such extremely low surface tensions are, however, only reached at high concentrations of fluorinated surfactants and only with highly specific structures. Since fluorinated surfactants are exceedingly expensive, it is imperative that the lowest surface tension is attained with the minimum quantity of surfactants.
The problem of attaining the lowest possible surface tension with fluorinated surfactants has been the subject of innumerable patents and publications, which detail specific and idealized structures having such properties.
In all cases the preferred candidate surfactants have distinctive and highly specific structures, which if varied even slightly drastically alter the attainable surface tensions. A fundamental reason that the attainment of a minimal surface tension at the lowest practicable use level is not easily answered is that the surface tension decreases as the fluorinated tail increases, while the solubility generally decreases so markedly when even one --CF.sub.2 -- group is added that precipitation of the sparingly soluble fluorosurfactant frequently occurs.
It has long been known that the surface tension of hydrocarbon surfactants, which at best is 26-27 dynes/cm, can be depressed to 23 dynes/cm with sparingly soluble alcohols. In fact, the adventitious nature of this effect is so marked that surface tension curves of conventional commercial surfactants frequently have minima unless the surfactant is scrupulously purified.
Bernett and Zisman, J. Phys. Chem, 65, 448 (1961), teach that synergistic mixtures of conventional hydrocarbon surfactants and fluorinated 1,1-dihydro alcohols can be prepared which attain low surface tensions with smaller concentrations of the fluorinated agent. The resultant solutions are, however, unstable and the fluorinated alcohols are, moreover, volatile and acidic. With the ammonium salt of a perfluoronoanoic acid, the fluorinated 1,1,-dihydroalcohols are not sufficiently soluble and eventually form gelatinous precipitates.